This invention relates generally to steam turbine systems for power generation and, more particularly, to structural support arrangements for parts of the system against seismic loading.
A steam turbine system includes pieces of equipment that are connected to the steam turbine itself by piping that carries the working fluid. Moisture separator reheaters (MSRs) are utilized in nuclear steam turbine cycles or systems to reduce turbine exhaust moisture and to increase thermal efficiency. The MSRs are connected between the high and low pressure turbines and are of such large size that they are spaced a distance from the turbines and are connected to these turbines by piping systems generally referred to as crossunder pipes that communicate from the high pressure turbine to the MSR and crossover pipes from the MSR to the low pressure turbine methods. Current practice includes methods to support an MSR and its piping in relation to the turbines as needed to satisfy differential thermal expansion criteria.
The turbines themselves are rigidly anchored in a direction transverse to the shaft to rigid supporting foundation structures, such as of concrete and steel. In one form of current practice, the MSRs are also rigidly anchored to their supporting foundation structures which are generally spaced from the turbine foundation. To accommodate differential thermal expansion, the piping system is provided with sufficient flexibility features, usually in the form of expansion joints. The flexibility features are required primarily to keep the piping reactions at the turbine and MSR connections within allowable limits in addition to maintaining acceptable levels of stress in the piping itself. The arrangement is satisfactory under most conditions. Upon the occurrence of a seismic event, however, it is likely that there will be relative displacement of the MSR and turbine foundations. This means that the pipes interconnecting the MSRs and turbines will be subject to large displacements at their end points. The magnitude of these displacements is such that it is not feasible to provide the crossunder and crossover pipe systems with sufficient flexibility features to maintain acceptable reaction levels at the turbine and MSR connections as well as acceptable stresses in the piping.
In another form of current practice, the MSRs are supported on their foundation such that they are free to move in a horizontal plane in response to the thermal piping reactions from the connecting crossover and crossunder pipes. This type of arrangement is known as a "floating" MSR system which offers the advantage of minimizing the flexibility features which would otherwise have to be provided for the piping to meet differential thermal expansion loading criteria. Freedom of movement in a horizontal plane is generally provided by hanging the MSRs by means of pivoting rods from cradle supports. This type of arrangement is also deficient, however, in meeting seismic loading requirements. Since the MSRs are not rigidly connected in the horizontal plane to their supporting foundations, they will tend to remain stationary while the MSR and turbine foundations are undergoing seismic loading displacements. The crossunder and crossover pipes, connecting the MSRs to the anchored turbines which are undergoing the same displacements as the turbine foundation, will attempt to displace the MSRs along with the displacement of the turbine foundation. The MSRs will thus impose very large inertia forces on the crossunder and crossover pipe connections to the turbines that is likely to exceed allowable limits.
Therefore, it is desirable in those applications where turbine systems may be seismically loaded to have a support system which can accommodate the seismic loading as well as differential thermal expansion.
By the present invention, an arrangement is made in which the turbines are rigidly supported on their foundation and the MSRs are provided with floating supports with additional elements in the combined arrangement for protecting the piping from seismic loading effects. Functional considerations would permit the use of cradletype supports for the floating MSR system. However, for economic and space considerations, the proposed arrangement of this invention utilizes sets of permanently lubricated sliding support plates located between the MSR support feet and the MSR foundation. In addition, there are a number of viscoelastic dampers attached to the MSRs. Viscoelastic dampers are generally known and are generally characterized by having a first member rigidly attached to the supported element (i.e., the MSR) and a second member rigidly attached to a foundation (in the present case to a steel plate joined to the turbine foundation) with a dampening substance, such as bitumen, between the first and second members that permits relatively free movement in any direction in response to gradual or light forces such as would be induced by thermal expansion characteristics of the piping, but which exhibits a substantially rigid characteristic upon occurrence of a large, sharp force as would be encountered due to a seismic shock. Suitable viscoelastic dampers for use in the present invention are those of the type that have been previously used in applications such as for mounting diesel engines on shipboard in which a central cylinder is attached to the supported engine and rests within an outer cylinder attached to the foundation with a viscoelastic dampening substance located therebetween. Articles of commerce sold under the name Gerb Viscodamper Vibration Isolation Systems are suitable for this purpose.